For example, as shown in FIG. 7A and FIG. 7B, an impeller 10a of a centrifugal compressor is fixed to a rotational main shaft of a centrifugal compressor not shown via a shaft hole 16 provided on an inner circumferential side, and is configured to include a disk 11 with one surface curved to be thinner to the tip, a cover 12 in a shape of facing the curved surface of the disk 11, and many blades 13 provided so as to section a space between the curved surfaces of the disk 11 and the cover 12 into a vortex shape.
Types of this impeller 10 include one called a three-piece type in which the disk 11, the cover 12, and the blades 13 are independently produced and mutually bonded for assembly and one called a two-piece type in which the cover 12 and the blades 13 (or the disk 11 and the blades 13) are integrally fabricated and bonded to the disk 11 (or the cover 12) fabricated independently therefrom. In both of the impellers 10 of the three-piece type and the two-piece type, bonding is performed by welding or brazing. Whether welding or brazing is performed for bonding is determined according to the size, strength, or others of the impeller 10. Note that the impeller 10 shown in FIG. 7A and FIG. 7B is of the two-piece type, illustrating an example in which the disk 11 and the cover 12 integrally fabricated with the blades 13 are bonded with a brazing part 14.
Bonding by brazing has a thermal cycle in which, with a brazing material made of, for example, an Au—Ni alloy, being interposed between members to be bonded (for example, between the disk 11 and the blades 13), the temperature is increased to a brazing temperature equal to or higher than the melting temperature of the brazing material, the state is held for a set period, and then cooling is performed.
Bonding by brazing has the following advantages.
Since the brazing temperature and the temperature in a solution heat treatment of a material (precipitation-hardening-type stainless steel) configuring the impeller 10 can be made within the same temperature range, the heat treatment for brazing and the solution heat treatment can be performed in a united manner.
Also, compared with the impeller 10 fabricated by welding, the impeller 10 fabricated by brazing deforms less, and is unbalanced less.
Furthermore, since a brazing process is performed under vacuum, the surface of the impeller 10 after the process is cleaned, a process of removing an oxide coating later can be eliminated, and dimensional accuracy required can be easily ensured.
An impeller manufacturing method by brazing with the advantages as described above is disclosed in Patent Document 1.
With a quenching process starting at a temperature slightly lower than that of the liquid phase of the brazing material, the strength of a brazed bonded part is insufficient, resulting in a crack in the brazed bonded part. An object of Patent Document 1 is to solve the problem of the conventional brazing method as described above.
Patent Document 1 suggests a brazing thermal cycle, a typical example of which is shown in FIG. 1 of Patent Document 1. In FIG. 1, an assembly body to be brazed is heated for approximately six hours to a temperature of the liquid phase or a liquidus temperature of the brazing material, that is, approximately 1850 Fahrenheit degrees (1010° C.), and that temperature is retained for about one hour. In the course of this temperature increase, a temperature of 1200 Fahrenheit degrees (650° C.) is retained for approximately one hour. Furthermore, the brazed assembly body is cooled for approximately two hours to approximately 1300 Fahrenheit degrees (704.4° C.), and then the temperature of the assembly body is lowered for one hour to a temperature of approximately 350 Fahrenheit degrees (176.7° C.) for gas quenching. Patent Document 1 states that, with this thermal cycle, the rotary wing assembly body does not exhibit thermally-induced distortion, the entire brazed bonded part is solid, and no crack occurs. Note that Patent Document 1 recommends JIS SUS630 as a stainless steel configuring each member of the impeller and an alloy containing 80% to 85% gold (Au) and 15% to 20% nickel (Ni) (this may be hereinafter abbreviated as a Ni—Au alloy) as the brazing material. In the specification of the present application, % means a percentage by mass.